Combined magnetic resonance positron emission tomography devices (MR-PET devices), as were recently proposed, allow for a combination of two imaging methods, which interact particularly well. Within the scope of combined MR-PET examinations, some pre-measurements of magnetic resonance images are however generally necessary.
On the one hand, a locally-resolved knowledge of the attenuation values of the tissue of the current patient, the so-called attenuation map, is needed in order to evaluate the results of a PET measurement. This information allows the PET data to be corrected accordingly during the evaluation. It is therefore known to produce an MR image recording, from which different tissue classes can be identified.
A known method for this is the so-called Dixon method for separating water and fat. This method relates to the small difference in the Lamor frequency of protons bound in fat and water. To this effect, this frequency difference is noticeable such that the phases of protons in the fat and in the water diverge. A distinction is made here in particular between the state in which the fat and water magnetization are in phase (“in-phase-condition”) and the state in which the phase difference amounts to exactly 180° (“opposed-phase-condition”). While in the first instance the signals mutually intensify within a voxel, in the second instance an attenuation takes place. With the Dixon methods, two echoes are now recorded after a single excitation, namely an image, in which the phase difference between the water and the fat signal amounts to zero and an additional image, wherein the phase difference between the water and the fat signal amounts to 180°. One image therefore corresponds to the addition of water and fat signals, the other image corresponds to a subtraction of water and fat signals. The fat/water ratio within a voxel can however be determined therefrom. This information is used as a basis to calculate the attenuation map.
A second premeasurement which is relevant in respect of the actual MR examination and has to be implemented is a measurement to determine the shim settings. A body introduced into the B0 field of a magnetic resonance facility interferes with the homogeneity of this field. It is therefore necessary to determine shim correction terms in order to reproduce homogeneity. It is known to record two data records within the scope of an MR image recording, in which water and fat are in the same phase. The difference in the phases of adjacent voxels is directly connected to the field differences and can then be used to determine the deviations from the homogeneity. Corresponding shim settings are in turn derived herefrom, for instance shim currents or shim coils.
The implementation of these two measurements is time-consuming and puts additional stress on the patient who is not permitted to move at all during the examination. A renewed double MR image recording must also take place for each position of the patient positioning couch.